Feature request - Payment Prorata

Hello,
I would like to present you a use case that I would need to have.
In an association I am working with, they have a product 'Trial period' whereby you pay for 2 months a fee to access the service. They have later the option to upgrade and take advantage of the service for the remaining of the year.

The basic yearly subscription starts from 1st of January and ends on 31st of December. If the customer uses the pass of two months from 1 st of January till 1st of March and then upgrade we want to be able to charge them only the ten months remaining.

Do you think this is something we could have with ZCheckout or it s better suited to be handled with ZSubscriptions ?

Many thanks
Victor M. da Silva